Thermally controlling fracturing



April 23, 1968 c. s. MATTHEWS E AL I THERMALLY CONTROLLING FRACTURING Filed Sept. 9, 1966 FIG.

FIG. 2

|NVENTORS:

C. S. MATTHEWS F. VAN MEURS C. W. VOLEK BY: d 7 0 THEIR AGENT United States Patent Office 3,379,250- Patentted Apr. 23, 1968 of Delaware Filed Sept. 9, 1966, Ser. No. 578,244 5 Claims. (Cl. 166-111) This invention relates to a process for heating a subsurface earth formation. More particularly, it relates to controlling the heating of fluid that is being circulated through a horizontal fracture that interconnects a pair of wells.

In practicing the present invention, a subsurface region is heated by employing heating fluid that is flowed into a horizontal fracture. Copending patent application Ser. No. 578,533, filed Sept. 12, 1966, describes a particularly suitable procedure for thermally biasing a subsurface earth formation in order to ensure the formation of a fracture that is horizontal. The process of the copending application can advantageously be combined with the present process, by thermally biasing a subsurface earth formation and extending a horizontal fracture through the thermally biased earth formation from one well to another by means of the process of the copending application, and then heating the region around the fracture to a selected temperature by means of the present process.

It is often desirable to heat a subsurface earth formation in order to facilitate the recovery of a material that can be thermally converted into a mobile fluid. For example, the recovery of a viscous oil or tar may be improved by heating the material in situ to a temperature at which its viscosity is relatively low. Similarly, a solid material such as sulfur may be melted in situ and recovered as a fluid, or an ore such as cinnabar may be heated in situ to a temperature at which a mineral such as mercury is released, or the like.

When a subsurface earth formation which is to be heated has a relatively low permeability, the earth formation is usually fractured so that an adequate rate of flow can be obtained in respect to a mobile fluid. When an earth formation is fractured in connection with a process for recovering a fluidized component, it is generally desirable to have the fracture extend along a substantially horizontal plane. Such a horizontal fracture provides a flow path which has a significant areal extent and can readily be extended into contact with a pair of wells.

In hydraulically fracturing a subsurface earth formation, a fluid is confined in a region which is bounded by the earth formation and the pressure on the fluid is increased until a fracture is produced in the earth formation. The pressure which causes a fracture in a subsurface earth formation or rock is called the fracturing pressure or fomation breakdown pressure. Where the fracture is horizontal, one layer of the earth formation is lifted above the other. This requires a lifting of the weight of the overburden in addition to the overcoming of the tensile forces which hold the layers of rock together. The separation between the walls of the horizontal fracture need only be a few tenths of an inch in order to provide for an adequately high rate of flow of fluid. Thus, an adequate rate of flow through such a fracture is provided by simply counterbalancing the weight of the overburden without appreciably bending the overlying strata in order to lift it by a significant amount. The fluid pressure required to counterbalance the weight of the overburden is called the overburden pressure. The tensile strength of a rock or subsurface earth formation is generally low and, in respect to a vertical fracture, the fracturing pressure may be considerably less than the overburden pressure. Where the fracture is horizontal, the difference between the fracturing pressure and the overburden pressure is generally only a few hundred pounds. For example, in a well that is 10,000 feet deep, the overburden pressure may be 10,000 p.s.i. with the fracturing pressure only 200 p.s.i. higher.

Where a horizontal fracture has been extended through a subsurface earth formation, it is often advantageous to mechanically prop open the walls of the fracture. In propping a fracture, various types of methods and apparatuses are used for depositing propping grains, such as sand grains, walnut shells, glass beads, or the like, between the walls of the fracture. Such propping grains will usually support the weight of the overburden when the fluid is being circulated through a horizontal fracture at a pressure which is less than the overburden pressure. Alternatively, a horizontal fracture can be hydraulically propped by keeping it full of fluid at a pressure which is at least equal to the overburden pressure. Where fluid is circulated through such a hydraulically propped fracture, the fluid injection pressure must be higher than the overburden pressure in order to overcome the flow resistance due to friction. If the injection pressure becomes greater than the fracturing pressure, new or divergent fractures may form within the subsurface earth formation.

in field operations, it has often been observed that difficulties are encountered when an attempt is made to circulate a hot fluid such as steam through a horizontal fracture. For example, in respect to a mechanically propped horizontal fracture through which cold water can be flowed at a suitable rate at a pressure which is less than the overburden pressure, when steam is injected into the fracture the pressure required to maintain the same rate of flow, or any flow, may rise to a pressure which is significantly greater than the overburden pressure. In such a situation, it is often dangerous to inject the steam at a pressure suflicient to cause it to flow. A new or divergent fracture is apt to be extended, in an upward direction. An upward extension is favored, since the compressive forces due to the weight of the overburden becoming less at shallower depths. In such a situation, where the pressurized fluid that is causing the fracturing is steam, once a fracture has started to form it is apt to be extended by the expansion of the compressed steam in the well conduits and the fracture even though no additional steam is injected into the well. Such upward extensions of fractures are apt to cause blowouts to the surface.

The objectives of the present invention are inclusive of the following:

It provides an eflicient process for heating a subsurface region of significantly large areal extent by heating fluid that is flowed into a fracture which extends along a generally horizontal plane.

It provides a process for circulating an aqueous fluid through a horizontal fracture that is encountered by a pair of wells while heating the fluid at a rate such that a significant rate of fluid flow can be maintained at an injection pressure which is not significantly greater than the overburden pressure.

It provides a process for circulating a heated aqueous salt solution through a horizontal fracture while correlating the rate at which the salt solution is softened, flowed, and heated in a manner that avoids a need for significantly increasing the injection pressure in order to maintain an injection pressure sutficient to sustain an adequate flow rate when the temperature of the salt solution is increased.

Additional objects and advantages of the invention will be apparent from the following descriptions and drawings. The descriptions and drawings are merely illustrative.

In general, in accordance with the present invention, the heating of a subsurface earth formation is accomplished by the following steps. Fluid communication is established between a pair of wells and a substantially horizontal fracture, using wells that encounter the fracture at points which are separated by a significant distance, such as at least feet. Fluid is pumped into the fracture in an amount sufficient to form a layer of fluid that extends be tween the wells. The fluid in that layer is heated to a temperature that is significantly greater than the normal earth formation temperature, such as at least about 100 F. greater than the formation temperature. While heating the fluid the temperature gradient within the layer of fluid between the wells is kept at a value which is relatively insignificant, such as less than about 1 F. per foot.

In a preferred embodiment of the invention, the fluid in the layer extend-ing between the wells is heated by heating fluid at the surface and circulating it through the fracture from one well to the other. Measurements are made of the temperatures at which the circulating heated fluid leaves one well and enters the other. The rate at which the fluid is circulated and the rate at which its temperature is increased are adjusted so that the difference between the temperatures at which the fluid leaves one well and enters the other is insignificant.

FIGURE '1 is a vertical section which shows an earth formation penetrated by an injection well and a production well and illustrates the effect of injecting steam into a relatively cold horizontal fracture.

FIGURE 2 is a vertical section which shows a similar earth formation and wells and illustrates the effect of injecting heated fluid into a horizontal fracture in accordance with the present invention.

Referring to FIGURE 1, injection well 1 and production well 2 illustrate wells completed in a conventional manner into a substantially impermeable subsurface earth format-ion 3. The wells are provided with casing strings 4 which extend through and are opened into the surrounding earth formation, the openings preferably being perforations 5. Well 1 is provided with an injection tubing string 6 and packer assembly 6a and well 2 is provided with a production tubing string 7 and packer assembly 7a. The injection and production tubing strings are connected to conventional types of heat-ing, pumping, storing, and oilrecovering units, not shown.

Wells 1 and 2 are interconnected by means of horizontal fracture 8. Where the regional tectonics are conducive to the formation of horizontal fractures, a fracture such as fracture 8 can be formed by injecting fluid through a tubing string assembly such as 6 until the injection pressure exceeds the formation breakdown pressure. The fluid is then injected at a relatively high rate in order to maintain a relatively high pressure until the fracture has been extended by radial expansion and has encountered another well, such as well 2, or has been extended into a zone into which another well will be drilled. Normally, such a fracture would be propped open by filling it with propping material such as particles 9.

In general, in the situation shown in FIGURE 1, a cold aqueous fluid can be flowed at a reasonable rate from an injection well such as 1 to one or more production wells such as 2 in response to an injection pressure that is less than the overburden pressure. However, when steam is injected, such a flow often becomes cut off at the pressure at which the water was flowed and a flow of steam can be induced only by applying a pressure that is significantly higher and is apt to fracture the earth formation. In many reservoirs, the application of such a high pressure during the injection of steam is dangerous. New fractures are apt to be formed and to be opened all the way to the surface. The resulting steam eruption may result in a loss of life as well as a loss .of the well.

When steam is injected into an earth formation, the steam is usually significantly hotter than the earth formation or rock, Whenever a rock is contacted by fluid that is significantly hotter than the rock, the rate at which heat is transferred from the fluid to the rock is relatively high. Thus, when steam is injected into a well such as well 1, the distribution of the heat that is transferred into earth formation 3 has the form shown by cross hatching 10.

The temperature to which the earth formation is heated is highest near the well, declines rapidly with radial distance away from the well and becomes substantially equal to the normal formation temperature within a distance of not more than 5 or 10 feet from the well.

It has been discovered that when a subsurface earth formation is heated, it expands by an amount that is significant with respect to a fracture within the formation. When the temperature of an earth formation is increased by a relatively small amount, such as 50 F., its dimensions may increase by several percent. When an earth formation is heated in the manner shown by the cross hatching 10, the hot zone is apt to contain a cylindrical portion, having dimensions such as a height of 10 feet and a diameter of 3 feet, in which the temperature has been increased by an amount suflicient to cause a vertical ex.- pansion amounting to five percent along the height of the cylinder. Such an expansion would cause the overlying materials to be lifted by six inches. The lifting would be done, in effect, by a piston having a surface area of only about seven square feet. Even where such heating is done at a relatively shallow depth, such a lifting would apply a high pressure to the rocks in the heated zone. In addition to lifting the weight of a three-foot column of the earth material above the heated zone, the lifting would necessitate the bending each of the overlying strata by an amount sufficient to permit a six-inch elevation at the well head. Such a pressure loading of the rocks in a small heated zone often causes fracture propping materials, such as particles 9, to become imbedded in the walls of the fractures and causes a fracture to become closed as the walls are pressed together in the manner illustrated at Ma.

Referring to FIGURE 2, in heating a horizontal fracture in accordance with the present invention, fluid is flowed into the fracture and heated in a manner such that the temperature gradient is low along the radial lines such. as the line that extends between the wells. This causes the heat which is transferred to the subsurface formation to be substantially uniform over an areally extensive disc-shaped zone as shown in section, by cross hatching 11. Even where the distance between the wells is as small as 25 feet, the area of of the cylindrical region in which the rocks are heated contains nearly 2000 square feet. In respect to such a large piston, the force required to :bend the overlying strata is insignificant in respect to the force produced by applying the overburden pressure to an area of such a size.

In a preferred procedure for practicing the present invention, the fluid which'is flowed into the horizontal fracture is heated at a surface location while it is being flowed through the fracture at a rate suflicient to transport heat from the surface to the fracture. Whenever a hot fluid is flowed from a surface to a subsurface location, heat is being transferred from the fluid to the surrounding materials all of the time the fluid is in the conduit that extends from the heating location to the subsurface location. In general, in a practical process in shallow to medium wells, the fluid flow rate should exceed a minimum of at least about barrel per minute and, in deeper wells, the flow rate should be higher. While a fluid is flowed into the fracture at an adequate rate, the inflowing fluid is heated to a temperature that is raised above the normal temperature of the subsurface earth formation. The increase in the temperature to which the fluid is heated can be continuous or intermittent, as long as the average rate of the increase is properly correlated with the average rate of fluid flow. The ratio of the rate at which the temperature of the inflowing fluid is raised above the temperature of the earth formation to the rate of the fluid flow should be kept low enough to maintain a low temperature gradient in radial directions extending away from the point of fluid injection. Such a temperature gradient should be generally less than about 1 F. per foot.

The suitability of the rate at which the temperature of the inflowing fluid is being increased can readily be checked by determining the temperature at which the fluid leaves an injection well, such as well 1, and enters a production well, such as well 2. The fluid temperature gradient along a line intercepting the wells is the difference between these temperatures divided by the distance between the Wells. The temperatures at which the fluid leaves one well and enters another can readily be measured by conventional types of downhole, surface located, temperature measuring devices. Where such measurements are made at surface locations, calibration measurements are desirable in respect to the amounts by which the temperatures are changed during the passages through the wells from and to the surface locations.

In general, the fluid which is heated and circulated through the fracture can comprise substantially any pumpable fluid. In certain situations, it may be desirable to initiate the heating of an oil-containing subsurface earth formation at a time at which little or no surface heating equipment is available at the well side. In accordance with the present invention, this can be accomplished by heating the fluid in situ. In respect to a fracture in an oil-bearing formation the fracture can be filled with a fluid which reacts exothermically with the oil in the walls of the fracture. Such a reaction tends to apply the same amount of heat to all of the layer of fluid that extends between the wells. Such a fluid is preferably a liquid that is reactive with a component of the earth formation. The spent reactive liquid and reaction products can be displaced with a fresh batch of reactive liquid, and the in situ heating operation can be repeated in order to impart additional increments of heat to the walls of the fracture. Oil-reactive liquids that can be used in such a process include hydrogen peroxide, nitrous oxide, sulfur trioxide, fluid mixtures containing such reactants, and the like. Where the chemical heating is employed as a pretreatment preceding the circulation of surface heated fluid through the fracture, it is important that the temperature of the first portions of surface heated fluid and the rate of increasing the temperature to which the fluid is heated be correlated with flow rate in order to maintain a specified low temperature gradient along the line extending between the Wells.

In a preferred procedure for practicing the present invention, an aqueous liquid is heated at a surface location and then circulated through its fracture at an injection rate at least greater than about barrel per minute. The initial portions are preferably injected at a temperature such that, by the time they reach the fracture, their temperature is not significantly greater than that of the fracture walls. The rate at which the temperature to which the inflowing portions are heated is then increased in the manner described above.

In numerous field locations it is advantageous to use a locally available aqueous solution, such as a brine obtained from a subterranean aquifer. The electolyte concentration of such solutions is often relatively high and relatively rich in alkaline earth metal salts. We have discovered that such brines can advantageously be used by pre-treating them with a water-softening treatment that is adapted to produce a brine which is soft in respect to scale-formation at the temperature to which the aqueous solution is heated, before they are heated. As known to those skilled in the art, such a water-softening can involve precipitating, exchanging, or chelating, the alkaline earth metal ions. Such a softening can advantageously be accomplished by means of the process described in U.S. Patent 3,193,009. That patented process provides a soft aqueous liquid which is rich in salts that remain soluble at the temperatures to which subsurface earth P formations are often heated in thermal processes for the production of oil.

Where the subsurface earth formation to be heated is a rock which remains mechanically competent at the temperature to which it is to be heated, the horizontal fracture through which heated fluid is circulated is preferably propped, initially heated with water, and subsequently heated with steam. During the water-heating operation, the water is preferably softened as required and heated at a rate correlated with the flow rate as described above. After the temperature within the fracture has reached about 212 F., the aqueous fluid which is being heated and pumped into the fracture is preferably converted to steam. The conversion is preferably done gradually in order to avoid any lowering of the temperature attained along the walls of the fracture. The hot aqueous liquid which is being pumped into the fracture is preferably mixed with steam in proportions that are increased at a rate causing substantially no decrease in the temperature of the fluid arriving at the production well. Such a gradual conversion avoids the possibility of cooling the fracture walls near the downstream portions of the flow path. Such a cooling would occur if, for example, a discrete slug of dry steam were to be interposed between portions of hot water. In such a situation, the condensation of the steam would cause a localized pressure reduction and the resultant evaporative cooling would produce a slug of fluid which would soon be cooler than the fracture walls. When the heating fluid is steam, it is generally economically advantageous to circulate the steam through a propped fracture at a pressure not signiz'icantly greater than the pressure of dry steam at the temperature to which the steam is being heated.

In heating a subsurface earth formation that is normally unconsolidated, or which tends to become unconsolidated at the temperature to which it is to be heated, the propping of a horizontal fracture has little or no value. In such a formation at the temperature to which the fracture is to be heated, the propping grains would become imbedded in the fracture walls and would allow the walls to close. In heating such an unconsolidated formation in accordance with the present process, the heating is started while keeping the fracture filled with liquid at a pressure sufiicient to support the weight of the overburden. A liquid is circulatcd While the inflowing portions are heated, in the manner described above, and the production wells are throttled as necessary in order to maintain a liquid pressure exceeding the overburden pressure. As long as the fluid being circulated consists of substantially noncompressible liquid, no significant risks are created if the injection pressure equals or rises above the fracturing pressure. If fracturing occurs, the extension of the fracture causes a drop in the pressure and the pressure reduction prevents any further extension of the fracture. A formation that is incompetent at the temperature to which it is to be heated is apt to (a) contain one or more components which become thermally mobilized when heated and (b) be converted to a permeable formation as the thermally mobilizable components are removed. In treating such a formation, the heating is preferably accomplished while circulating liquid at a pressure exceeding the overburden pressure and, after suflicient material has been extracted from the fracture walls to provide permeable channels along the walls, the fracture is allowed to close. When the fracture is closed, after a permeable channel has been formed along its walls by extracting a mobilized component from the walls, the liquid heating fluid is preferably converted to steam that is circulated at a relatively low pressure. Such a conversion preferably employs the gradual conversion procedures described above.

Example I.Fracture closure problem A five-spot pattern of injection and production wells was completed into a shallow layer of Missouri tar sand. At its normal formation temperature, this tar sand is substantially impermeable because of the high viscosity of the tar. A horizontal fracture was extended from the injection well to the production wells at a depth of 300 feet. This fracture formed at from about 800 to 1000 p.s.i. and cold aqueous liquid could be circulated through it at a suitable rate at a downhole injection pressure of 350 p.s.i.g. Several attempts to circulate steam made it clear that whenever steam was pumped into this well, as soon as the steam reached the reservoir formation, the pressure required to sustain any flow rose rapidly to a pressure at least about as high as the fracturing pressure. The same thing occurred in a fracture which was propped by depositing conventional propping grains between the fracture walls.

Example Il.Scaling problem A similar pattern of injection and production wells was completed and horizontally fractured in a nearby portion of the reservoir described in Example I. Cold water could be circulated through this fracture at about five barrels per minute at a bottom-hole pressure of 410 p.s.1.g.

The water that was circulated was obtained from a nearby source well. When this water was heated, by mixing it with steam, and then circulated through the fracture, the pressure required to maintain the circulation rate soon began to rise. Various remedial treatments, including the propping of the fracture, failed to eliminate the necessity of increasing the injection pressure in order to maintain an adequate rate of flow.

It was discovered later that the scale-depositing tendency of this water increased with temperature and, at about 200 F the hot water deposited scale at a relatively high rate. By the time the temperature of the water being circulated through the fracture had been raised to about 210 F. the bottom-hole injection pressure had become as high as 750 p.s.i.g. The need for applying such pressures proved to be due to the tubing strings and the fracture having become tightly plugged with deposited scale.

Example III.-Successful fracture heating The plugging effects of the scale deposits were reduced, in respect to circulating a fluid through the tubing string and fracture, by a conventional well acidization treatment. When the circulating and heating of water was resumed, the water used was the water mentioned in Example 11. This water amounts to an aqueous solution of salts inclusive of alkaline earth metal salts. The water was softened by a process of the type described in US. Patent 3,193,009 and was softened to the extent required to provide a liquid that deposited no scale at the temperature to which it was heated.

The initial portions of softened water were injected at about 200 F. and the temperature was increased at a rate of about 50 per day. The water temperature increasing rate and water-flowing rate were such that the temperatures of the fluid at leaving an injection well and entering a production well that was 25 feet away were never separated by more than about 25 F. These temperature-increasing and fluid-circulating rates were maintained until the temperature of the water was 475 F. At this time, the bottom-hole injection pressure was about 600 psig.

Example I V.Cnversi0n from hot water to steam While the hot water was being circulated through the fracture at 475 F. as described in Example 111, the rate at which the water was being circulated through the heater was gradually throttled back until the rate was cut in half. This rate reduction was affected over about a threeday period. The rate of this reduction caused a longer residence time in the heater and thus, without any significant change in the temperature or the pressure at which the hot fluid was circulated through the fracture, the circulating fluid was gradually converted from hot water to 8 a steam. The quality of the steam was gradually increased from about zero percent to about 60 percent.

We claim as our invention: 1. A process for heating an areally extensive portion of subsurface earth formation, which process comprises:

(a) establishing fluid communication through a pair of Wells and a substantially horizontal fracture that is located within a substantially impermeable subsurface earth formation and is encountered by the Wells at locations which are separated by at least about 25 feet;

(b) flowing suflicient fluid into the fracture to form a layer that extends between the boreholes of the pair of wells; and

(c) heating the layer of fluid that extends between the wells to a temperature at least about 100 F. above the normal temperature of the earth formation while maintaining a temperature gradient of less than about 1 F. per foot along the layer of fluid extending between the wells.

2. The process of claim 1 wherein the layer of fluid that extends between the wells is heated by:

(a) heating fluid to increasingly high temperatures at a surface location;

(b) circulating the heated fluid through the fracture at a rate that is at least greater than about one-tenth barrel per minute; and

(c) adjusting the fluid heating rate relative to the fluid circulating rate as required in order to maintain said low temperature gradient within the fracture.

3. The process of claim 2 wherein:

(a) the heated fluid is an aqueous liquid; and

(b) the aqueous liquid is softened as required in order to provide an aqueous liquid that is nonscaling at the temperature to which the liquid is heated.

4. The process of claim 2 wherein:

(a) the substantially impermeable earth formation is competent at temperatures exceeding 212 R;

(b) the horizontal fracture is mechanically propped;

and

(c) the layer of fluid that extends between the wells is heated to a temperature of at least about 212 F. by circulating heated aqueous liquid through the fracture and is further heated by circulating steam through the fracture.

5. The process of claim 1 wherein:

(a) the substantially impermeable subsurface earth formation is an oil-containing earth formation;

( b) the fluid which is flowed into the fracture to form a layer extending between the wells is an oil-reactive fluid that reacts exothermically with oil; and

(c) the layer of fluid that extends between the wells is initially heated in situ by a reaction between oil in the fracture walls, and oil-reactive fluid within the fracture and is subsequently heated by heating fluid at a surface location and circulating the heated fluid through the fracture.

References Cited UNITED STATES PATENTS CHARLES E. OCONNELL, Primary Examiner.

IAN A. CALVERT, Assistant Exam irzer. 

1. A PROCESS FOR HEATING AN AREALLY EXTENSIVE PORTION OF SUBSURFACE EARTH FORMATION, WHICH PROCESS COMPRISES: (A) ESTABLISHING FLUID COMMUNICATION THROUGH A PAIR OF WELLS AND A SUBSTANTIALLY HORIZONTAL FRACTURE THAT IS LOCATED WITHIN A SUBSTANTIALLY IMPERMEABLE SUBSURFACE EARTH FORMATION AND IS ENCOUNTERED BY THE WELLS AT LOCATIONS WHICH ARE SEPARATED BY AT LEAST ABOUT 25 FEET; (B) FLOWING SUFFICIENT FLUID INTO THE FRACTURE TO FORM A LAYER THAT EXTENDS BETWEEN THE BOREHOLES OF THE PAIR OF WELLS; AND (C) HEATING THE LAYER OF FLUID THAT EXTENDS BETWEEN THE WELLS TO A TEMPERATURE AT LEAST ABOUT 100*F. ABOVE THE NORMAL TEMPERATURE OF THE EARTH FORMATION WHILE MAINTAINING A TEMPERATURE GRADIENT OF LESS THAN ABOUT 1*F. PER FOOT ALONG THE LAYER OF FLUID EXTENDING BETWEEN THE WELLS. 